Currently available intrusion detection sensors/systems employed, for example, in the field of vehicle interior protection, suffer from false triggering and false alarms. These may be caused by footballs or similar objects hitting exterior surfaces, loose stones thrown up from passing vehicles, deliberate triggering of the alarm by rocking or striking the vehicle, thunderclaps, door-knocks from adjacent vehicles, etc. This often becomes more of a problem as a vehicle ages and interior trims and fittings become worn and loose. An intrusion sensor fitted to the vehicle may interpret these movements and vibrations as genuine intrusions.
In order to be sure that the sensor detects even small, slow moving ‘real’ intrusions into the protected vehicle, high sensitivities, i.e. high amplification gains, are required. This high sensitivity often degrades the false alarm immunity.
The functioning of a currently available motion detector used for monitoring the interior of a vehicle (or, say, other spatial areas such as rooms) can be described as follows: an air-ultrasonic carrier signal is projected into, and reflected around, the space that is to be protected. This signal is subsequently phase and/or amplitude modulated by any intrusion into the protected space, and any other “normal” motion of the vehicle as described above, before being received back at the ultrasonic receiver. “Normal” motion of the vehicle body can quite easily trigger the alarm, and such false alarms can be the cause of great annoyance to the general public.
As a result, many UK and foreign legislative and insurance rating bodies have included stringent checks for such problems in their vehicle certification tests. Non-compliance may cause a vehicle to either be considered unsuitable for sale, or to receive a higher than expected insurance rating.
Thus, a need exists for a method and means of improving the performance and reliability of motion detection systems, especially as regards false triggering of a detection system.